I tried this for a while and I consider the experiment failed.
No closed source means no Whatsapp, etc. Let's face reality, no-one is using XMPP ;)
In the end I started the policy "Prefer open-source software on Android", which works pretty well:
* Note taking using orgzly
* Tasks using opentasks (https://github.com/dmfs/opentasks)
* File Sync using Syncthing
* Contacts, Calendar using davdroid & nextcloud
* Firefox for browsing (this is a no-brainer)